This invention relates to fluid meters and, more particularly, to a fluid meter of the internal gate rotary vane type.
Rotary vane-type fluid meters with an internal sealing gate generally exhibit excellent performance characteristics compared to other types of rotary positive displacement meters (such as the lobed impeller or external sealing gate type meters). As a general rule, the reason for better performance is better fluid flow through the meter and lower friction of the moving parts.
However, in attempting to further refine rotary vane-type meters with internal gas sealing, it has been found that existing designs could be improved with new inventions and solutions based on the results of mathematical/computer iterations and simulations, extensive empirical research testing, and experience with current embodiments. It is an object of this invention to improve capacity, reduce turbulence and compressive/suction cycles, reduce the pressure differential for a given rating, improve the fluid flow for minimum restrictions, reduce the effect of friction, and improve stall torque characteristics for such a meter.
Fluid meters have exacting requirements for minimum performance. For a given full capacity rating, a meter must not exceed some standard of maximum pressure drop, or differential, across the meter connections (as this is a measure of its lack of friction and flow impediments). For gaseous rotary meters this standard is presently one inch water column (1/27 psig) at full capacity on natural gas (0.6 S.G.) where the inlet is at seven inches water column (1/4 psig) over atmospheric pressure. As some pressure differential would normally occur across a pipe of equal length, connection to connection, such a requirement dictates low friction of mechanism and minimal fluid flow impediments. It follows that designs having lower mechanical friction and fewer flow impediments have a higher capacity and thus more commercial value.
Another measure of fluid meter performance is accuracy of measuring actual volume from low flow rates to capacity. While 100% accuracy is desirable at all flow rates, it is recognized as being impossible. Accordingly, industry standards use a minimum level of performance which allow some deviations in accuracy. In the United States for gaseous rotary meters this standard presently is a band of .+-.1% around 100% accuracy for flow rates which the meter must meet during many years of operation without calibration, at all rated pressures, and in all conceivable ambient temperatures. Therefore, a meter with minimal friction and fewer flow impediments is more likely to meet accuracy requirements given such operating conditions.
Higher pressure operation frequently requires special considerations in meter design as the change in fluid density can have substantial effects on accuracy of actual fluid flow measurement. Typical solutions are ratings for a specific range of pressures (where gear ratios of output are altered to normalize the accuracy curve within acceptable limits) and flow shaping (such as flow jetting vanes). Such solutions are an engineering compromise which complicates product design, manufacture, and marketing, and are thus to be avoided if possible.
Another measure of fluid meter performance is a term called "rangeability". Rangeability is defined, for gaseous meters, as the ratio of full flow rate divided by that lower flow rate which falls out of the accuracy band of 100% .+-.1%. Rangeability is expressed as a ratio (such as 20:1 which would mean the meter's accuracy was falling below 99% at 5% of full flow). This performance criteria is a very sensitive measure of the meter's mechanical friction and/or freedom from compression/suction cycles as these cause the rotating components to try to operate slower than the gas velocity, which results in blowby at the seals. Rangeability can also be a measure of the sealing effectiveness (seal blowby at a given differential), but mechanical friction and/or compression/suction cycles cause the increased pressure differential to drive fluids through the seal.
The above concepts are typically charted for clarity for commercial marketing purposes. FIG. 1 illustrates a typical performance chart for a gaseous rotary meter. In FIG. 1, by convention, the highest accuracy values cannot exceed 101% (see Points B and C) and the lowest accuracy values cannot be lower than 99%, including compression frequencies (see Point D) and "boost" or "droop" at full capacity (Point E is a "droop", F is a "boost"). The Rangeability of this example is 20:1 (or 100%.div.5%, the point at which the accuracy curve falls below 99%, Point G). Also, the flange-to-flange pressure differential cannot exceed 1.0" H.sub.2 O (see Point H) for 7.0" H.sub.2 O inlet pressure.
In the example of FIG. 1, a rotary meter having a high operating pressure differential due to mechanical friction or flow impediments would result in the capacity being lowered until the 1.0" H.sub.2 O differential were met. A rotary meter with substantial compressive cycles might not even qualify to the standard. A rotary meter with high rotational velocity friction (due to such items as geared gate driving mechanisms, bearings, lubricating baths, and seals), or flow rate related impediments, might have excessive "droop" as to limit capacity. A rotary meter whose accuracy is adversely affected by pressure might not qualify. A rotary meter having high tare friction (and possibly poor sealing) might substantially reduce rangeability.
Accordingly, it is a primary object of this invention to reduce mechanical friction is an internal gate rotary vane fluid meter.
It is another object of this invention to increase driving torque.
It is a further object of this invention to reduce compression/suction cycles of the rotating components.
It is yet another object of this invention to reduce fluid flow impediments.
It is still another object of this invention to reduce the effects of gas density.